Implantable cardiac stimulation devices are well known in the art. Such devices may include, for example, implantable cardiac pacemakers and defibrillators either alone or combined in a common enclosure. The devices are generally implanted in a pectoral region of the chest beneath the skin of a patient within what is known as a subcutaneous pocket. The implantable devices generally function in association with one or more electrode(-s) carrying leads which are implanted within the heart. The electrodes are positioned within the heart for making electrical contact with the muscle tissue of respective heart chamber. Conductors within the leads connect the electrodes to the device to enable the device to deliver the desired electrical therapy. Hence, cardiac rhythm management devices are implantable devices that provide electrical stimulation to selected chambers of the heart in order to treat disorders of cardiac rhythm. Common conditions for which pacemakers are used are the treatment of bradycardia, where the ventricular rate is too low, and heart failure.
A programmable electronic controller of the device causes pacing pulses to be output in response to lapsed timing intervals and sensed electrical activity (i.e. intrinsic heart beats not as a result of a pacing pulse). Pacemakers sense intrinsic cardiac electrical activity by means of electrodes disposed near the chamber to be sensed. A depolarization wave associated with an intrinsic contraction of the atria or ventricles that is detected by the pacemaker is referred to as an atrial sense or ventricular sense, respectively. In order to cause such a contraction in the absence of an intrinsic beat, a pacing pulse (either an atrial pace or ventricular pace) with energy above a certain pacing threshold is delivered to the chamber via the same electrode or via other electrodes than used for sensing the chamber.
Bi-ventricular pacing provides therapy options for a patient suffering from heart failure. However, new challenges have been presented by placement of the left-ventricular lead via the coronary sinus in bi-ventricular pacing systems. Due to the proximity of the coronary veins to the phrenic nerve, left ventricular pacing may result in undesirable phrenic nerve stimulation. The left phrenic nerve, which provides innervation for the diaphragm, arises from the cervical spine and descends to the diaphragm through the mediastinum where the heart is situated. As it passes the heart, the phrenic nerve courses along the pericardium, superficial to the left atrium and left ventricle. Because of its proximity to the electrodes used for pacing, the nerve can be stimulated by a pacing pulse. The resulting involuntary contraction of the diaphragm can be annoying to the patient and may also interfere with breathing.
Accordingly, there exist various methods and devices for detecting and reducing phrenic nerve stimulation of cardiac pacing systems within the art.
In U.S. Pat. No. 7,392,086 to Sathaye methods involving delivery of pacing pulses, sense of transthoracic impedance signals following the pacing pulses and analysis of deviations in the transthoracic impedance signals are disclosed. More specifically, a transthoracic impedance signal is analysed in a time window following a left-ventricular pace pulse, e.g. 500 milliseconds long time windows starting at the delivery of a left-ventricular pulse, by comparison with a transthoracic impedance signal resulting from an additional pulse delivered during a cardiac refractory period of the left ventricle to find deviations indicating phrenic nerve stimulation. A threshold test for the available particular pacing vectors may be performed to find and select the best vector in terms of desirable energy levels and reduced phrenic nerve stimulation.
In U.S. Patent Application No. 2010/0305638 to McCabe et al, methods for phrenic nerve activation detection and phrenic nerve activation avoidance are disclosed. According to these methods, impedance is used to identify portions or phases of respiration of interest for detection of phrenic nerve stimulation. Accelerometer signals or other vibration signals are used to detect diaphragmatic response due to phrenic nerve stimulation. The detection is performed within a detection window initiated based on delivery of a left ventricular pulse during a respiration phase of interest.
In U.S. Patent Applications Nos. 2003/0065365 and 2005/0060002 both to Zhu et al., a cardiac rhythm management device in which an accelerometer is used to detect diaphragmatic or other skeletal muscle contraction associated with output of a pacing pulse are disclosed. Upon detection of diaphragmatic contraction, the device may adjust pacing pulse energy and/or pacing configuration.
However, there is still a need within the art for improved methods and devices for detecting phrenic nerve stimulation and for reducing the presence of phrenic nerve stimulation.